The present invention relates to a mobile radiocommunications equipment offering its user the possibility of having confidential calls with his correspondent.
"Confidential call" is here understood to be a call in which the voice transmission signals exchanged are ciphered using a method and/or ciphering keys which are known only to the correspondents or to an organisation grouping those correspondents.
The present invention is particularly intended to allow such confidential calls in large geographic zones.
The present surge in public cellular radiotelephony networks offers an attractive possibility because of the numerous existing base stations which provide a very large territorial cover.
Certain of the cellular networks allow the ciphering of voice transmission calls. This is the case in particular of the European GSM system and its derivatives. However, the cipherings carried out generally ensure the confidentiality of the call only on the air interface. Even in the case of a GSM call between two mobile terminals, the voice transmission signals are deciphered and transcribed (from 13 to 64 kbits/s) for the links between various entities of the infrastructure on the ground. The security of calls is not therefore complete. Furthermore, the ciphering functions are under the control of the network operator and not of the users, which is not satisfactory for providing confidentiality in the above sense.
An object of the present invention is to propose a radiocommunications equipment which is suitable for confidential calls by means of the existing structure of a public mobile telephony network. It is also desired that the equipment should be discreet in its use and that it should furthermore be of moderate cost price.